"This doesn't seem physically possible!!"
Ok. First of all I want to say to all of you guys who say that Bungie has nothing to do with the books, you might want to rethink your thinking. Bungie & Microsoft have EVERYTHING to do with the books. The 5th Halo book, Contact Harvest was written by Joseph Staten, the main writer for all 4 halo games.
OK?!
Back on topic though. In The Fall of Reach , Master Chief and ALL of the Spartan IIs (except for Grey team that is otherwise engaged somewhere in the galaxy) are getting ready to leave Reach on a mission to try and capture a Covenant vessel, take it to High Charity, kidnap one of the Hierarchs, and bring him back to force a treaty. They get stopped by a little crisis of the fact that a massive Covenant fleet has shown up to pwn Reach. The UNSC holds them off for a while, but the fleet is too massive, and getting bigger all the time. Meanwhile, during this attack, the Spartans split up with MC (SIERRA-117), James, and Linda, go to secure a UNSC corvette with venerable NAV data, the rest of the Spartans, lead by Fred, go to secure the ground-based generators for the orbital MAC guns. Chief and Linda succeed in destroying the corvette, but James dies, and Linda is almost killed. They make it back to the Pillar of Autumn to make a run for it, while the Covenant starts glassing Reach. Meanwhile on the planet, Fred and the others had to jump and high-altitude and high-speed from their pelican, and most of the Spartans die or are seriously wounded. But they fight for a while, and the Covenant glass most of the planet while they are fighting. The part of the planet they are on though, does NOT get glassed.
Any way to make a long story slightly shorter, I believe that Reach will likely take place PRIOR to Halo 1, and will switch back and forth between MC and the ground-based squads.